Method and system for monitoring aircraft status

ABSTRACT

A computerised method is provided for monitoring the status of an aircraft. The method comprises sending, from an output module of an aircraft monitoring system, a reporting more report data signal response and monitoring, at an input module of the aircraft monitoring system, for received data signal to the aircraft monitoring system. This sets a contract with the aircraft that may be used to monitor for unexpected behaviour or non-compliance with the contract terms. The method further comprises determining, at a processor of the aircraft monitoring system, if one or more alert criteria have been satisfied by the received data signal responses and generating, at an alerting module of the aircraft monitoring system, an alert based on the determination.

FIELD OF THE INVENTION

The present invention relates to a method and system for monitoring thestatus of an aircraft. In particular, the invention relates to a methodand system for alerting based on monitored communications that areexpected to be received from an aircraft.

BACKGROUND TO THE INVENTION

Current aircraft monitoring systems typically use aircraftcommunications addressing and reporting system (ACARS) data incombination with radar data in order to track the progress of aircraft.This data may be used by air traffic controllers or alternativelyprovided as a service to aircraft operators.

In the ACARS system, each aircraft is fitted with a VHF transceiver forproviding a data link between the aircraft on-board equipment and groundequipment. This data link may be provided through a direct transmissionfrom the aircraft to a ground station, or alternatively the aircraft maytransmit the data to a satellite, which then forwards the data to asatellite ground station. These transmissions are received at the groundstations by a data link service provider that then routes the data tothe air traffic controllers or aircraft operators.

The periodicity within which a given aircraft will emit ACARS datatransmissions is configured by the operating airline and is typically inthe order of ten to twenty minutes. This is generally determined inorder to provide a balance between receiving up to date data and the permessage costs associated with the data transfer. This periodicity is setby appropriately programming the on-board avionics during maintenance ofthe aircraft and cannot be changed during a flight.

In view of this relatively long period between consecutive ACARS messagetransmissions, significant distances can be covered by an aircraftbetween the transmissions, which can in turn lead to an uncertainty inthe estimated position and path of an aircraft. Furthermore, the timestamp for any given ACARS transmission is only accurate to within aminute and the position data is reported within an accuracy of threedecimal places.

If the aircraft is forced to circle in a given area of airspace, forexample, in an airport holding pattern, this will not be immediatelyapparent from the ACARS data as the aircraft will likely have performeda full circle by the time a subsequent ACARS transmission is carriedout. This can lead those monitoring the ACARS data to be unsure as towhether these data transmissions are erroneous or if the aircraft trulyhas remained in a given area of airspace between subsequent ACARStransmissions.

Increasing the standard frequency (i.e. reducing the period betweenconsecutive transmissions) of ACARS messaging, as programmed into theaircraft's avionics during maintenance, would provide a more up to dateset of position data. However, if each airline were to do this as astandard across the board then a large burden would be placed on theACARS network, since it is a one-to-one digital data link system. Thismay overload the network and reduce its reliability and accuracy.

Airspace across the world is split up into a number of three-dimensional(3D) blocks of space known as sectors. Each sector has one or more airtraffic controllers that communicate with and are responsible for thesafety of aircraft operating in, or about to enter, that airspacesector. These controllers work for Air Navigation Service Providers(ANSPs) and are trained to manage the aircraft such that there is a safeand orderly flow of aircraft from point to point in the most efficientmanner.

In order to achieve this, air traffic controllers communicate withaircraft to give active support and authorisations as well as to receiveinformation from the aircraft. Usually this communication is carried outover voice radio such as radio transmissions in the VHF or HF bands. Oneissue with voice radio is that only one transmission can be made on agiven frequency at a given time and so, even if there is a strong radiosignal, transmissions may be cut off or become unintelligible. In orderto ensure that transmissions are accurately received, it is necessary toread back the communication, which also increases the time it takes fora given communication to be completed.

Furthermore, voice communications can be subject to misunderstandings orlanguage barriers, voice quality can be low, and VHF voice frequenciesare subject to high traffic congestion. To combat these negativeaspects, a committee was set up to establish a new system, the FutureAir Navigation System (FANS), to improve these communications, forexample by using a data link system to encapsulate messages between theANSP and the aircraft.

A number of standard format communications have been determined than canbe used to send common commands such as level or altitude assignments,crossing constraints, lateral deviations, route changes and clearances,speed assignments, radio frequency assignments, and various requests forother information, with the option of a free-text message forcommunications that fall outside of the standard list of common commandsor responses.

These communications are commonly known as a Controller-Pilot Data LinkCommunications (CPDLC) and they eliminate the need to validatecommunications by multiple transmissions and reading back as bothparties can see the communication in text form and the communicationsare available on demand such that they can be easily reviewed later orprinted.

These data link messages are commonly encapsulated and transmitted usingthe Aircraft Communications Addressing and Reporting System (ACARS)protocol. Aircraft using ACARS may be fitted with a VHF and/or HFtransponder for providing a data link between the aircraft on boardequipment and ground station equipment. This data link may be providedthrough a direct transmission from the plane to the ground, oralternatively through a microwave transmission via a satellite. Thesetransmissions are received at the ground by a data link service providerand then routed to aircraft operators by the data link service providerfor a charge per message. Messages transmitted from the aircraft to aground system may be referred to as downlink messages and messagedtransmitted from a ground system to the aircraft may be referred to asuplink messages.

Another aspect of FANS is the ability to set up an Automatic DependentSurveillance Contract (ADS-C). ADS-C uses the FANS avionics systems thatare a part of the on-board Flight Management Systems (FMS) of FANSequipped aircraft to automatically provide information such as theaircraft's position, altitude, speed, intent and meteorological data tousers such as ANSPs or airlines. At a minimum the ADS-C message willcontain three-dimensional position information, the timestampcorresponding to the position information and a figure of merit (FOM)that indicates the accuracy of the position data.

The contract is defined by the ground system of the end user and mayindicate that communications should be sent from the aircraft to the enduser's ground systems in response to specified periodic, or demandbased, or event based criteria or a combination of these criteria. Up tofive separate ground systems are able to maintain ADS contracts with agiven aircraft and, currently, these ADS-C connections are typicallyused by the air traffic controllers of ANSPs that have FANS enabledground systems to reduce the reliance on voice channel dialogues betweenthe pilot of an aircraft and the air traffic controller, which in turnreduces the workload of both the controller and the pilot and allows theseparation between respective aircraft to be reduced.

The ANSP may determine the data link capabilities of a given aircraft byexchanging Air Traffic Service Facilities Notification (AFN) messageswith the aircraft. These messages may also include the addressinformation that allows a subsequent FANS session to take place.

In the past, airlines have relied on the reports issued by air trafficcontrollers and only used passive means for monitoring the status oftheir aircraft. It has been appreciated by the applicant that aproactive system for airlines to monitor the status of their aircraftthat can be developed quickly and implemented using an aircraft'sexisting equipment is desirable.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a computerised method isprovided for monitoring the status of an aircraft. The computerisedmethod comprises sending, from an output module of an aircraftmonitoring system, a reporting contract request to the aircraft'savionics, the reporting contract request defining one or more reportcriteria upon which the aircraft's avionics are required to provide adata signal response to the aircraft monitoring system; receiving, at aninput module of the aircraft monitoring system, the data signalresponses sent from the aircraft's avionics to the aircraft monitoringsystem; determining, at a processor of the aircraft monitoring system,if one or more alert criteria have been satisfied by received responses;and generating, at an alerting module of the aircraft monitoring system,an alert based on the determination.

Advantageously, this embodiment provides a method whereby a reportingcontract, such as an ADS contract, may be initiated with an aircraft andintelligent alerting may be provided based on the reporting contractmessages.

Preferably, the report criteria may define a first interval at which theaircraft's avionics are required to provide a data signal response, thealert criteria may define a second interval and an alert may begenerated, at the alerting module of the aircraft monitoring system, inthe absence of a required data signal response being received during thefirst or second interval. This advantageously provides a method that mayautomatically alert to the user that an expected periodic reportingcontract response message has not been received. The user may theninvestigate the situation to determine if further action is needed.

Optionally, the report criteria may indicate that an immediate datasignal response is required from the aircraft's avionics and the alertmay be generated, at the alerting module of the aircraft monitoringsystem, in the absence of a data signal response being received withinan interval defined by the alert criteria. Furthermore, the one or morereport criteria or the one or more alert criteria may be received from acriteria database and the data signal responses may comprise aircraftlocation information corresponding to the aircraft.

In one embodiment, the input module is configured to receive flight plandata and ACARS data corresponding to the aircraft and an alert isgenerated, at the alerting module, if it is determined that a flightphase change message corresponding to the aircraft has been received butflight plan data corresponding to the aircraft has not been received.This provides an alert if an aircraft is about to or has taken off butthe flight plan corresponding to that aircraft's flight has not beenreceived by the system. This is desirable because the absence of flightplan data for the aircraft will mean that some other alerting functionsmay not be available for the flight until a relevant flight plan data isprovided.

The computerised method may include receiving, at the input module,flight plan data corresponding to the aircraft and generating an alert,at the alerting module, if the aircraft location information isdetermined to deviate vertically or laterally from the flight plan databy a given amount. This will alert the end user to a change in course ofthe aircraft that may require further investigation to determine if theunexpected behaviour of the aircraft is of concern. This aids users whomay be monitoring a large number of aircraft by bringing their attentionto specific aircraft that may need consideration in view of theunexpected change in the flight path.

Furthermore, the method may optionally comprise receiving, at the inputmodule, Controller-Pilot Data Link Communications (CPDLC) messagescorresponding to the aircraft and generating an alert by the alertingmodule if the deviation from the flight plan data is determined, by theprocessor, not to be authorised in the content of the CPDLC messages.This enables the method to determine if an unexpected deviation is theresult of correspondence between the controller and the pilot, in whichcase the alert may not be necessary, or if further investigation isstill required. This aids a user by reducing the number of alerts thatmay be generated, and need to be reviewed, in situations where acontroller has authorised or instructed a change in the aircraft's routethat would otherwise be unexpected.

The computerised method may generate an alert, at the alerting module,if the aircraft is determined or estimated to intersect a given regionof airspace based on current aircraft location information or flightplan data. This region of airspace may be determined by a user selectionor by a weather alert. Advantageously, this allows the computerisedmethod to automatically alert the end user if the aircraft is estimatedto have entered, or have already entered, undesirable airspace, such asa war zone, a region containing a volcanic ash cloud or another badweather region.

Preferably, an alert is generated, at the alerting module, if it isdetermined, at the processor, that a received data signal response is anemergency report or a connection denial message. The reporting contractrequest and the corresponding data signal responses preferably conformto an Automatic Dependent Surveillance Contract.

In one embodiment, the computerised method further comprises receiving,at the input module, Controller-Pilot Data Link Communications (CPDLC)messages corresponding to the aircraft storing the CPDLC messages in adata store and providing the CPDLC messages to a user upon a userrequest. The alerts that have been generated may also be stored in adata store and are provided to a user upon a user request. Thisadvantageously helps a user to establish if the event that led to thegeneration of the alert was discussed between the pilot of the aircraftand an ANSP controller.

The computerised method according to the first aspect of the inventionmay further comprise receiving, at the input module, Air TrafficServices Facilities Notification (AFN) messages corresponding to theaircraft; determining, at the processor, if one or more AFN messagecriteria have been satisfied by the received AFN messages; andgenerating, at the alerting module, an alert based on the determination.

Alternatively, a computerised method may be provided for monitoring thestatus of an aircraft comprising receiving, at the input module, AirTraffic Services Facilities Notification (AFN) messages corresponding tothe aircraft; receiving, at the input module, aircraft locationinformation corresponding to the aircraft; determining, at a processor,if one or more AFN message criteria have been satisfied by the receivedAFN messages; and generating, at the alerting module, an alert based onthe determination and the aircraft location information.

Preferably, the processor may determine if AFN message criteriaidentifying a time period for receiving a FANS logon confirmationmessage have been satisfied and cause an alert to be generated, at thealerting module, if the FANS logon confirmation message is not receivedafter the aircraft has been in FANS enabled airspace for the identifiedtime period. This will highlight to an end user, such as an airlineoperator, that there has been an unexpected loss of contact betweentheir aircraft and the FANS ground system of an ANSP.

Optionally, the processor may determine if AFN message criteriaidentifying a time period for receiving a FANS logon confirmationmessage have been satisfied and cause an alert to be generated, at thealerting module, if the FANS logon confirmation message is not receivedwithin the identified time period from an AFN contact advisory messagehaving been sent to the aircraft. This advantageously highlights to anend user that a FANS handover between two ANSPs has not been successfuland that further investigation of the aircrafts status may be desirablein order to eliminate any gaps between ANSP tracking of the aircraft.

According to a second aspect of the invention, a system is provided formonitoring the status of an aircraft. The system comprises an outputmodule configured to send a reporting contract request to the aircraft'savionics, wherein the reporting contract request defines one or morereport criteria upon which the aircraft's avionics are required toprovide a data signal response to the system; an input module configuredto receive data signal responses sent from the aircraft's avionics tothe system; a processor configured to determine if the one or more alertcriteria have been satisfied by received data signal responses; and analerting module configured to generate an alert based on thedetermination.

This advantageously provides a system that can initiate a reportingcontract, such as an ADS contract, with an aircraft and provideintelligent alerting based on the criteria of the reporting contract andthe content of the reporting contract messages that are, or are not,received.

Preferably, the report criteria defines a first interval at which theaircraft's avionics are required to provide a data signal response, thealert criteria defines a second interval and the alerting module isconfigured to generate an alert in the absence of a required data signalresponse being received during the first or second interval defined bythe criteria such that a user is automatically alerted that an expectedperiodic reporting contract data signal response message has not beenreceived. Optionally, the report criteria may indicate an immediate datasignal response is required from the aircraft's avionics and thealerting module may be configured to generate an alert in the absence ofa response, within an interval defined by the alert criteria.

In a preferred embodiment, the data signal responses that the inputmodule is configured to receive comprise aircraft location informationcorresponding to the aircraft. Furthermore, the input module may befurther configured to receive flight plan data and ACARS datacorresponding to the aircraft and the alerting module may be configuredto generate an alert if it is determined that a flight phase changemessage corresponding to the aircraft has been received but flight plandata corresponding to the aircraft has not been received. This providesan alert if an aircraft is about to or has taken off but the flight plancorresponding to that aircraft's flight has not been received by thesystem. This is desirable because the absence of flight plan data forthe aircraft will mean that some other alerting functions may not beavailable for the flight until a relevant flight plan data is provided.

Optionally, the input module may be configured to receive flight plandata corresponding to the aircraft and the alerting module may beconfigured to generate an alert if the processor determines that theaircraft location information indicates that the aircraft has deviatedvertically or laterally from the flight plan data by a given amount.

Preferably, the input module is further configured to receiveController-Pilot Data Link Communications (CPDLC) messages correspondingto the aircraft and the alerting module is further configured togenerate the alert only if the deviation from the flight plan data isdetermined by the processor to not be authorised in the content of theCPDLC messages. In this manner, the number of alerts that are generatedby the system may be reduced such that the end user can focus on thegenerated alerts that may require further investigation to verify thestatus of the monitored aircraft.

Where the input module is configured to receive flight plan datacorresponding to the aircraft, the alerting module may be configured togenerate an alert if the processor determines or estimates that theaircraft will intersect a given region of airspace based on the flightplan data. The definition of the given region of airspace may bereceived from a user selection or may be a bad weather alert.

The alerting module of the system may be configured to generate an alertif the processor determines that a received data signal response is anemergency report or if the data signal response to the reportingcontract request is determined by the processor to be a connectiondenial message.

The input module of the system may be further configured to receiveController-Pilot Data Link Communications (CPDLC) messages correspondingto the aircraft and the processor may be configured to store the CPDLCmessages in a data store such that they can be provided to a user upon auser request. This enables a user investigating an alert to obtainadditional information regarding the context of the aircraft's statusaround the time that the alert conditions were noted.

Preferably, the reporting contract requests that the output module isconfigured to send and the corresponding responses that the input moduleis configured to receive will conform to an Automatic DependentSurveillance Contract. The system may further comprise a data store,wherein the processor is further configured to store the alerts thathave been generated in the data store and to provide the alerts thathave been generated to a user upon a user request.

The input module of the system according to the second aspect of theinvention may further be configured to receive Air Traffic ServicesFacilities Notification (AFN) messages corresponding to the aircraft andthe alerting module may be configured to generate an alert if theprocessor determines that an AFN confirmation message has not beenreceived when expected based on the aircraft location information.

Alternatively, a system for monitoring the status of an aircraft may beprovided comprising an input module configured to receive Air TrafficServices Facilities Notification (AFN) messages corresponding to theaircraft and aircraft location information corresponding to theaircraft; a processor configured to determine if one or more AFN messagecriteria have been satisfied by the received AFN messages; and analerting module configured to generate an alert based on thedetermination and the aircraft location information.

Preferably, the AFN message criteria identify a time period forreceiving a FANS logon confirmation message and the alerting module isconfigured to generate an alert if the FANS logon confirmation messagehas not been received after the aircraft has been in FANS enabledairspace for the identified time period. This will highlight to theuser, such as an airline operator, that there has been an unexpectedloss of contact between their aircraft and the FANS ground system of anANSP.

Optionally, the AFN message criteria may identify a time period forreceiving a FANS logon confirmation message and the alerting module maybe configured to generate an alert if the FANS logon confirmationmessage has not been received within the identified time period from anAFN contact advisory message having been sent to the aircraft. Thisadvantageously highlights to an end user that a FANS handover betweentwo ANSPs has not been successful and that further investigation of theaircrafts status may be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a system according to anembodiment of the invention interacting with the respective datasources;

FIG. 2 is a schematic representation of a system according to anembodiment of the invention;

FIG. 3 is a message flow diagram of the process of establishing a FANSsession with an ANSP;

FIG. 4 is a message flow diagram of the process of handing over a FANSsession from a current ANSP to a next ANSP;

FIG. 5 is a flow diagram showing the main steps performed by anembodiment of the invention;

FIG. 6 is a flow diagram showing additional steps that may be performedby an embodiment of the invention; and

FIG. 7 is a flow diagram showing the main steps performed by analternative embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 shows an aircraft 10 that is equipped with a FANS avionics systemto provide a digital data link between the aircraft 10 and one or moreusers on the ground. The digital data link may transmit messagesdirectly to one or more ACARS ground stations 12, which may then beforwarded through a communications network 14. The communicationsnetwork may include one or more of a local area network (LAN), a widearea network (WAN), the internet, a mobile telephony communicationsystem or a satellite communication system. Alternatively, the aircraftmay transmit the ACARS messages via a satellite link 16; in this case,the ground station would be a satellite ground station 18.

The ACARS messages that are forwarded through the communications network14 are then sent on to, and collected centrally by, a system 20. TheseACARS messages may include FANS messages sent over the ACARS protocol aswell as other ACARS messages, such as ACARS aircraft locationinformation reports and OOOI messages as will be described in moredetail below. The system 20 is for monitoring the status of an aircraft.Typically the status of the aircraft will be monitored during a flightof the aircraft. In this context, a flight is taken to include groundbased activities, such as taxiing, from the moment the aircraft systemsare powered up at the origin location to the moment that the aircraftsystems are powered down at the destination location. The communicationsnetwork 14 may also be used to route communications between the aircraft12 and an Air Navigation Service Provider (ANSP) system 21, or an AirTraffic Controller (ATC) system associated with an ATC facility.

The communications network 14 may be any public, private, wired orwireless network and may comprise any suitable infrastructure, includingcopper cables, optical cables or fibres, routers, firewalls, switches,gateway computers and edge servers. The ground stations 12 may compriseVHF ground stations or HF ground stations that operate on the VHF or HFradio frequency ranges respectively. The term “ground station” is usedherein to refer to any receiver station at ground level. For theavoidance of doubt, these ground stations may include receivers locatedon ocean platforms, such as oil rigs, or floating vessels, such astankers or aircraft carriers.

The system 20, according to a first aspect of the invention, will now bedescribed in further detail with reference to FIG. 2. The system 20comprises a processor 22 that is connected to a criteria database 24, anoutput module 26, and an input module 28, via respective data lines. Theoutput module 26 is configured to couple to the communications network14 of FIG. 1, via a connection 26 a, such that uplink messages outputfrom the output module 26 may be communicated to the aircraft 10 via oneor more ground stations 12 or one or more satellite ground stations 18using a satellite link 16.

Similarly, the input module 28 is configured to couple to thecommunications network 14 of FIG. 1, via a connection 28 a, such thatdownlink messages output from the aircraft 10 may be received at theinput module 28 via one or more ground stations 12 or one or moresatellite ground stations 18 using a satellite link 16. The systemfurther comprises an alerting module 30 that is connected to theprocessor 22, via a further data line, and configured to output one ormore alerts to an end user. The alerting module 30 may be coupled to theend user's end user system 31 via a connection 30 a.

Optionally, the input module 28 of the system 20 may further be coupledto a flight plan store 32 via a connection 32 a. In this regard, it willbe appreciated that the input module 28 may be configured to receiveinput messages from a plurality of sources and that the messages fromeach source may be received at a separate input submodule, with therespective input submodules collectively forming the input module 28.The system 20 may optionally further comprise a data store 34. In oneembodiment, the data store 34 may be connected to the processor 22 by adata line.

In order to initiate an ADS contract with a given aircraft 10, theprocessor 22 may send instructions to the output module 26 uniquelyidentifying the aircraft and indicating at least one report criteria.The one or more report criteria may be retrieved by the processor 22from the criteria database 24, or alternatively the report criteria mayhave been provided by the end user system 31, for example via thealerting module 30 and the connection 30 a.

The criteria database 24 may be configured to store one or more reportcriteria for being used to determine the type of ADS contract thatshould be established with a given aircraft and to configure theparameters of the contract, such as the frequency of the downlinkreports. These report criteria may be default report criteria for agiven fleet or type of aircraft, or they may be report criteria thatrelate specifically to a particular flight or specific aircraft and thathave previously been provided by the end user system 31.

The report criteria may specify that the ADS contract is a periodiccontract, a demand contract or an event contract. A periodic contractallows the end user to specify the time interval at which the aircraft'savionics systems are required to send an ADS-C message reportinginformation regarding the status of the aircraft. The status of theaircraft may comprise information identifying the location of theaircraft, its speed and direction. In one embodiment, this interval maybe between 1 second and 4,096 seconds (i.e. approximately 68 minutes)and in a further embodiment the interval may be between 64 seconds and4,096 seconds. This interval may be altered during the flight to providemore frequent position information, for example during a given segmentof the flight or in the event that concerns are raised regarding thesafety of the aircraft.

In one embodiment, when the output module 26 has received the ADScontract details for a given aircraft, the output module 26 may bearranged to process the ADS-C data in order to translate and encapsulatethe ADS-C data, which is typically in a bit-orientated data format, inaccordance with the ACARS character-oriented communications protocol,which is typically not directly compatible with FANS data. Theencapsulated ADS contract details may then be output from the outputmodule 26 as a reporting contract request message via connection 26 aand transmitted to the aircraft's avionics via the communicationsnetwork 14 and the one or more ground stations 12 or one or moresatellite ground stations 18 using a satellite link 16. At theaircraft's avionics, the encapsulated message may be translated backinto a bit-orientated format and passed to the aircraft's FMS forhandling.

The aircraft's FMS can then record the details of the ADS contract andsend a contract acknowledgement to the system 20. The aircrafts FMS maythen also send the first ADS-C report as a data signal response, inresponse to the new contact, to the system 20. The data signal responseis encoded as a data signal so that is can be transmitted over the datalink network. The acknowledgement and the first report may betransmitted in a single downlink message, or alternatively as separatedownlink messages from the aircraft's FMS to the system 20 via the oneor more ground stations 12 or one or more satellite ground stations 18using a satellite link 16 and the communications network 14. Thisdownlink message may also be encapsulated in the ACARS communicationsprotocol for transmission.

The downlink messages will be received at system 20 by the input module28 via connection 28 a and the input module 28 may then transmit themessages, i.e. the data signal response, to the processor 22. In oneembodiment, the input module may translate the encapsulated messageprior to transmitting the message to the processor 22, such that theprocessor 22 can read the contents of the message. The processor 22 maythen store data corresponding to the received data signal response inthe data store 34. The data store 34 may be a volatile memory buffer ora cyclic buffer, or alternatively the data store 34 may be anon-volatile memory, such as a hard disk, floppy disk, magnetic tape,solid state drive, storage area network or optical discs.

The processor 22 may then compare the content of the received messagewith one or more alert criteria held in the criteria database 24 todetermine if one or more of the alert criteria have been met. The alertcriteria may correspond to the report criteria, as will be discussed infurther detail below.

The alert criteria may specify that, in the event that the processor 22determines that the received message is a connection denial message, theprocessor 22 should cause the alerting module 30 to generate an alertand the alerting module 30 may be configured to output the alert to anend user system 31 via connection 30 a. The alerting module 30 mayprocess the alert to format the alert message such that it is readableby the end user system 31 and encapsulate the alert in a header packetthat includes the appropriate destination information for the end usersystem 31 prior to outputting the alert.

The processor 22 may be configured to retry the ADS-C connection adefined period of time after receiving a connection denial message andthis may be configured to repeat a set number of times or until acontract acknowledgement is received. The defined period of time may beset by an instruction from the end user system 31. In one embodiment,the alert criteria may specify that the system 20 will retry the ADS-Cconnection without generating an alert after receiving a firstconnection denial message from the aircraft 10. In such an embodiment,the alert criteria may specify that the alert may then be generated if asecond or further connection denial message is received from theaircraft 10.

The report criteria may, for example, specify that a periodic contractdata signal response should be sent from the aircraft 10 to the system20 every 60 seconds. In this manner, the report criteria set up anagreement between the aircraft 10 and the aircraft monitoring system 20regarding when a data signal response will be sent from the aircraft 10to the aircraft monitoring system 20. Similarly, the alert criteria mayspecify that an alert should be generated if a data signal response isnot received within the period identified by the periodic contractreport criteria.

When the processor 22 receives the first data signal response from theaircraft 10, the processor 22 may initiate a timer. Upon receiving asubsequent data signal response, the processor 22 may reset the timerback to zero. In the event that the processor 22 determines that thetimer has reached 60 seconds and a subsequent data signal response hasnot been received, the processor 22 may determine that the alertcriteria (and the report criteria) have not been satisfied. Accordingly,the processor 22 may cause the alerting module 30 to output an alert.This alert may be communicated to the end user system 31 via connection30 a.

A demand contract allows a single, one-off, ADS-C periodic report, ordata signal response, to be requested from a given aircraft 10 inaddition to any periodic contract that is currently being maintained.This type of demand contract report will typically only be requested inresponse to a specific request received from the end user system 31, forexample if the end user system 31 wants to know where a given aircraft10 is at that moment in time. This may be useful in situations whereinthe current periodic contract interval is a comparatively long time andthe next periodic contract response is not expected for some time.

An event contract indicates to the aircraft's FMS that an ADS contractdata signal response should be transmitted from the aircraft 10 to thesystem 20 whenever a specific event occurs. For example the event mayinclude a waypoint change event occurring to the next and/or next butone waypoint in the FMS. This would occur due to a flight plan change ora change in a waypoint sequence and any waypoint changes would benotified to the system 20 until the event contract is cancelled.

Alternatively, the specific event may be a level range deviation alert,a lateral deviation alert or a vertical rate change alert. Each of thesealerts will only be triggered once and accordingly the system 20 mustset up a new periodic contract if a further alert of the same type isdesired. The level range deviation alert is triggered when the altitude(or flight level) of the aircraft extends above an upper limit flightlevel or below a lower limit flight level, these flight level limits aretypically defined in the ADS-C event contract.

The lateral deviation alert will be triggered when the actual aircraftlocation exceeds a given off-route distance threshold from the expectedposition of the aircraft 10 as defined in the active flight plan for theaircraft; the threshold will typically be defined in the ADS-C eventcontract. The vertical rate change alert will be triggered when thepositive or negative vertical rate of climb or descent is greater than agiven threshold.

The applicant has appreciated that these ADS contracts and associatedreports may be used by an airline to improve the monitoring of theairline's aircraft 10. Using the above ADS contracts, the system andmethod of the present invention may provide advanced alerting regardingthe status of a monitored aircraft. As stated above, the system maygenerate an alert if a response required by a ADS-C periodic eventcontract is not received within the appropriate interval of time definedby the report and alert criteria.

In one embodiment, the processor 22 may allow an additional period oftime in excess of the defined interval prior to determining that theperiodic report criteria have not been satisfied and generating analert. For example, if the report criteria of the ADS-C periodic eventcontract defines that a response message should be sent from theaircraft 10 to the system 20 every 60 seconds, the alert criteria mayspecify the processor allows the timer to reach 70 seconds since theprevious response message without the next response message having beenreceived prior to causing the alerting module 30 to generate an alert.In this manner the alert criteria may allow for an additional 10 secondperiod. This additional period may be a second interval of 10 secondsthat starts at the expiry of the first period (i.e. the 60 second periodin the above case), or alternatively it may be a second period that runsconcurrently with the first period (i.e. a 70 second period in the abovecase).

This additional time period will allow the system 20 to reduce falsealerting and account for small variations in the transmission time ofeach message from the aircraft 10 to the system 20. This additionalperiod of time may be a fixed period, or alternatively the additionalperiod may be a percentage of the interval defined by the ADS-C periodicevent contract.

Similarly, if the system 20 sends an ADS-C demand event contract requestto a given aircraft 10, then the system 20 may be configured to generatean alert if a response to the ADS-C demand event contract is notreceived within a given period of time. For example, this period of timemay be 10 seconds, 20 seconds, 1 minute or any other desired timeperiod.

In addition to receiving ADS-C data signal response messages, the system20 may be configured to receive other ACARS messages from the aircraft10 via the communications network 14 and the input module 28. Forexample, the system 20 may receive standard ACARS position reportsand/or ACARS reports regarding changes to the major flight phases fromthe aircraft 10. These major flight phases are typically referred to asOOOI events and may be used to indicate that the aircraft 10 is “Out ofthe gate”, “Off the ground”, “On the ground” or “Into the gate”. For theavoidance of doubt, the input module 28 of the aircraft monitoringsystem may also be configured to receive aircraft location informationfrom non-ACARS sources, such as radar data or Automatic DependentSurveillance Broadcast (ADS-B) data. In embodiments wherein the inputmodule 28 of the system 20 is coupled to the flight plan store 32 via aconnection 32 a, the system may also be configured to receive a computerflight plan for a given aircraft's flight.

If the processor 22 determines that the aircraft 10 has deviated fromthe flight plan by more than a defined amount then the processor 22 maybe configured to cause the alerting module 30 to generate an alert. Thisdefined amount could be a fixed value or a percentage value. Thedeviation may be a vertical/flight level deviation (for example, 200feet or a change of 2 in flight level) and/or a horizontal/lateraldeviation (for example, 5 nautical miles). The deviation may becalculated by the processor 22 by comparing aircraft locationinformation received from the aircraft 10 with flight plan data receivedby the system 20 from the flight plan store 32.

Alternatively, the processor 22 may determine that a flight level(vertical) or lateral (horizontal) deviation has occurred by the receiptof an ADS-C event contract response message indicating a level rangedeviation event or a lateral deviation event. The level range deviationevent and lateral deviation event data signal responses will bedetermined based on the aircraft active flight plan that is stored inthe aircraft's FMS. The aircraft active flight plan may be modifiedin-flight and accordingly it may vary in comparison to a computer flightplan received by the system 20 from the flight plan store 32. Therefore,deviation alerts may be generated by the system 20 based on the computerflight plan data without an ADS-C event contract response messageindicating corresponding deviation, or vice versa. These deviationalerts can enable users to quickly identify aircraft that may have beensubject to a re-routing and assess any knock-on effects that this mayhave on the arrival time of the aircraft and any future flights that theaircraft or its crew are scheduled to embark on.

In one embodiment, the input module 28 may be configured to receiveCPDLC messages corresponding to a given aircraft 10 via thecommunications network 14 and the connection 28 a. These CPDLC messagesare typically communications between a controller of an ATC service orANSP and the pilot of the aircraft 10. However, where thesecommunications are transmitted over the communications network 14,copies of these CPDLC messages may be forwarded by the communicationsnetwork 14 to the system 20.

In such an embodiment, the processor 22 may receive the CPDLC messagescorresponding to the aircraft 10 and process the CPDLC messages todetermine if a vertical or horizontal deviation of the aircraft 10 hasbeen authorised by a controller in the content of the CPDLC messages,for example if there has been an instruction to climb or descend. If theprocessor 22 determines that the deviation has been authorised, then theprocessor 22 may cause the alerting module 30 to output an indicationwith the deviation alert message to indicate that the deviation wasauthorised by a controller. Alternatively, the processor 22 maydetermine that an alert should not be generated by the alerting module30 if a deviation has been authorised by the controller.

The content of CPDLC messages typically follow a standard format andaccordingly the processor 22 may be configured to parse this informationand determine the meaning of the messages. The processor 22 may alsocause the received CPDLC messages to be stored in the data store 34 forarchiving or later retrieval. For example, these CPDLC messages may beprovided to the end user system 31 by the alerting module 30 upon therequest of a user of the end user system 31.

In one embodiment, the processor 22 may be configured to cause thealerting module 30 to output the content, or a summary of the content,of recently received CPDLC messages with any alerts that are generatedby the alerting module 30. In such an embodiment, the system 20 may sendthe alert to the end user system 31 with the accompanying CPDLC messagessuch that an end user of the end user system 31 may determine manuallyif a deviation was authorised in the CPDLC messages. In this case, itmay not be necessary for the processor 22 to parse the CPDLC messages orto determine the meaning of the messages.

Preferably, the system 20 may enable a user to define an area ofairspace bounded by a shape or polygon. The alert criteria and theprocessor 22 may then cause the alerting module 30 to generate an alertif a given aircraft's location is determined to enter, or be within, thearea bounded by the shape. Alternatively, the alert criteria and theprocessor 22 may cause the alerting module 30 to generate an alert ifthe computer flight plan or aircraft active flight plan is determined tointersect the area bounded by the shape.

The shape may be defined by clicking on a series of points on a map, orentering a series of coordinates; these points may then be used tocreate the outline or boundary of the shape and the shape may further beassociated with a given flight level or range of flight levels in orderto define a given volume of airspace. The shape may optionally beassociated with a start date and time and/or an end date and time. Thealert criteria and the processor 22 may cause the alert to be cancelledor reset in the event that the aircraft 10 is determined to have exitedthe area associated with the shape or if the flight plan is amended suchthat it no longer intersects the area bounded by the shape.

Alternatively, the shape may be a standard region or airspace, such as aFlight Information Region (FIR) and/or an Upper Information Region (UIR)or it may be defined by an outside source, such as a weather layer froma weather service. The weather layer may indicate an area of bad weathersuch as a volcanic ash alert, icing conditions or turbulence etc.

If the processor 22 determines that a flight phase change message (suchas an out of the gate or off the ground OOOI event message) has beenreceived from an aircraft 10, but that a computer flight plancorresponding to the current flight of the aircraft 10 has not beenreceived from the flight plan store 32 then the alert criteria and theprocessor 22 may cause the alerting module 30 to generate an alert. Thisis desirable because, in the absence of a computer flight plan for theflight, some of the other alerting functions will not be functional asthey require the processor 22 to compare received aircraft locationinformation with the received computer flight plan in order to determineif an alert should be generated.

A received computer flight plan may include an estimated time ofdeparture (ETD) and an estimated time of arrival (ETA) for the relevantflight of the aircraft 10. If the processor 22 determines that the ETAhas expired but that an on the ground OOOI event flight phase changemessage has not been received from the aircraft 10 then the processormay cause the alerting module 30 to generate an alert. This may indicateto the end user that an aircraft 10 is late and that furtherinvestigation or remedial action may be necessary. In one embodiment,the processor 22 may only cause the alerting module 30 to generate analert after an additional period of time has expired, with theadditional period of time commencing at the ETA. For example, an alertmay be generated if it is determined that the aircraft if 5 or moreminutes late in landing.

If a revised ETA is received from the aircraft 10 during the flight thenthis revised ETA may be used in place of the computer flight plan ETA.Furthermore, if no ETA is received for a given flight of an aircraft 10but an off the ground OOOI event flight phase change message has beenreceived, then the processor 22 of the system 20 may determine an ETAfor the flight by taking the time offset between the scheduled time ofdeparture and the timestamp of the off the ground OOOI event message andapplying the time offset to the scheduled time of arrival.

ADS-C communications also support emergency alerting, whereby a periodicreport that is tagged as an emergency report is transmitted from theaircraft 10 to any connected ground systems as a data signal response.This function may be triggered by the crew manually by selecting theADS-C emergency function or indirectly by triggering a different type ofalerting system in the aircraft avionics. If the processor 22 determinesthat an ADS-C data signal response received at the input module 28 is anemergency report, the alert criteria and the processor 22 may cause thealerting module 30 to generate a corresponding alert to be output to theend user system 31.

The alert criteria and the processor 22 may be configured to only causethe alert to be generated if more than a set number of emergency reportsare received, or if the emergency reports are received for more than agiven duration of time. This may help to identify (and optionallyignore) transient alerts that may have been triggered accidentally.

If the processor 22 determines that the aircraft has completed a 360degree turn, the alert criteria may specify that the processor 22 maycause the alerting module 30 to generate an alert to indicate that theaircraft is potentially in a holding pattern. This alert may be clearedby the processor 22 if it is determined that no further 360 degree turnshave been detected within a further period of time, for example 5minutes. This type of alerting would require a relatively shortreporting interval such that the 360 degree turn can be accuratelyplotted. Optionally, the system 20 may receive additional aircraftposition data input, such as Automatic Dependent Surveillance Broadcast(ADS-B) position reports. Optionally, a diversion alert may be generatedif a diversion message corresponding to a monitored aircraft 10 isreceived by the system 20.

If the processor 22 determines that an aircraft 10 that is on the groundand on departure, i.e. an out of the gate OOOI event flight phase changemessage has been received but an off the ground OOOI event flight phasechange message has not been received and that the aircraft 10 hasexceeded a defined speed and then slowed below that speed, then thealert criteria and the processor 22 may cause the alerting module 30 togenerate an alert. This alert may indicate that a rejected take off hasoccurred. Similarly, if processor 22 determines that an aircraft 10 hasdescended below a defined altitude (for example, 10,000 feet) and thenclimbed above the defined altitude by more than a defined amount (forexample, 1000 feet), then the alert criteria and the processor 22 maycause the alerting module 30 to generate an alert to indicate that ishas been necessary for the aircraft 10 to “go around”.

If the ATC or ANSP that monitors the airspace that an aircraft 10 is inis FANS enabled then the ATC or ANSP may have an active data linksession with the aircraft 10. Typically, a FANS connection should beestablished around 30 to 45 minutes prior to entering the airspace ofthe FANS enabled ANSP.

The process of establishing a FANS session with an ANSP will now bedescribed with reference to FIG. 3. FIG. 3 illustrates a timeline ofmessages between an ANSP system 21 and an aircraft 10. This processinforms the relevant ANSP of the datalink capabilities of the aircraft10 and enables the ANSP to correlate the aircraft connection with afiled flight plan.

When no other FANS connection has been established between the aircraft10 and a previous ANSP, the ANSP system 21 and the aircraft 10preferably exchange Air Traffic Service Facilities Notification (AFN)messages. This message is the means by which an aircraft 10 mayintroduce itself to the ANSP so that the ANSP is aware of the aircraft'sregistration number and the digital data link applications that theaircraft 10 supports. Initially, the pilot causes the aircraft 10 tosend an AFN Contact message 40 to the ANSP system 21, the ANSP systemwill then reply to this message with an AFN Acknowledgement message 42.The AFN Acknowledgement message 42 may also be described as a FANS logonconfirmation message.

The ANSP system 21 will then initiate a CPDLC connection by sending aConnection Request message 44 to the aircraft 10, to which the aircraftwould typically respond with a Connection Confirm message 46. Once theconnection has been established and is active, the bi-directionalexchange of CPDLC messages 48 may be carried out. If the connection islost, then the complete logon procedure must be repeated in order tore-establish the connection.

The communications network 14 may be configured to forward these AFNmessages to the system 20 and the input module 28 may be configured toreceive these AFN messages. Furthermore, the processor 22 may beconfigured to determine if AFN message criteria have been satisfied bythe received AFN messages and cause the alerting module 30 to generatean alert based on the determination. For example, the AFN messagecriteria may specify that a given region of airspace is monitored by aFANS enabled ANSP and that the aircraft is expected to have an activeFANS session with the ANSP whilst in or approaching that region ofairspace. In this manner, the AFN message criteria may serve to specifywhen given AFN messages are to be expected and under what conditions,such as the absence of an expected AFN message, an alert should begenerated.

If the processor 22 determines that an AFN acknowledgement message 42(FANS logon confirmation message) has not been received after theaircraft 10 has been in FANS enabled airspace for a time periodidentified by the AFN message criteria then the processor 22 may causethe alerting module 30 to generate an alert. This alert would highlightto the user that an aircraft 10 had not established an active FANSsession with an ANSP when it is expected to.

As the aircraft 10 approaches the boundary between two FANS enabledANSPs, the current ANSP will instruct the aircraft avionics that aconnection is to be established with the next ANSP. This process will bedescribed with reference to FIG. 4, which shows the current ANSP system21 initiating the process by sending a Next Data Authority message 50 tothe aircraft 10, wherein the message identifies the next ANSP system 21a. The current ANSP system 21 may then send an AFN Contact Advisorymessage 52 to the aircraft 10, which may then be acknowledged by theaircraft 10 with an AFN response 54.

The aircraft 10 may then send an AFN Contact message 40 to the next ANSPsystem 21 a and receive an AFN Acknowledgement message 42 from the nextANSP system 21 a. When the aircraft 10 has received the AFNAcknowledgement message 42, it will typically send an AFN Completemessage 56 to the current ANSP system 21 to inform the current ANSP thatthe AFN process has been completed with the next ANSP. The current ANSPmay continue to monitor the aircraft 10 until the aircraft is close tothe sector boundary of the airspace monitored by the current ANSP, atwhich point the current ANSP may send an End Service message 58 to theaircraft 10. Optionally, the aircraft 10 may then send Wilko andDisconnect messages 60 to confirm that the CPDLC connection with theANSP has been terminated.

The next ANSP may initiate a CPDLC connection by sending a ConnectionRequest message 44 to the aircraft 10, to which the aircraft wouldtypically respond with a Connection Confirm message 46. Once theconnection has been established and is active, the bi-directionalexchange of CPDLC messages 48 may be carried out between the aircraft 10and the next ANSP system 21 a.

If the processor 22 determines that an AFN acknowledgement message 42(FANS logon confirmation message) has not been received within a giventime period of the current ANSP sending an AFN Contact Advisory message52 to the aircraft 10 then the AFN message criteria and the processor 22may cause the alerting module 30 to generate an alert. The time periodmay be identified by the AFN message criteria. This alert wouldhighlight to the user that an aircraft 10 had not established an activeFANS session with a next ANSP when it is expected to.

A second aspect of the invention provides a computerised method formonitoring the status of an aircraft that will now be described withreference to FIG. 5. The method comprises sending 62 a reportingcontract request defining one or more report criteria from an outputmodule 26 to the aircraft's avionics. The reporting contract request maydefine one or more report criteria upon which the aircraft's avionicsare required to provide a data signal response to an aircraft monitoringsystem. These data signal responses are then monitored for and received64 at an input module 28, of the aircraft monitoring system, from theavionics of the aircraft 10. The reporting contract will typically be anADS contract and the report criteria associated with the reportingcontact request may be periodic, event or demand based criteria. Themethod further comprises determining 66 at a processor 22 if one or morealert criteria have been satisfied by received data signal responses.For example, the report criteria may define a periodic interval at whichthe aircraft's avionics are required to provide a response and an alertmay be generated 68 if the processor 22 determines that a requiredresponse has not been received during an interval defined by one or morealert criteria. As described above, the interval defined by the alertcriteria may be the same as or longer than the periodic interval definedby the report criteria.

Preferably, the responses comprise aircraft location informationcorresponding to the aircraft 10. Furthermore, the one or more reportcriteria and the one or more alert criteria may be received from acriteria database 24. The computerised method may comprise determiningif an ACARS flight phase change message, such as an ‘out’ or ‘off’ OOOIevent message, has been received for a given aircraft's flight andwhether flight plan data has been received for that flight. If it isdetermined by the processor 22 that a flight phase change message hasbeen received but that corresponding flight plan data has not bereceived then the method may comprise generating an alert to bring thisto the attention of the end user system 31 or an end user. This enablesthe user to become aware that an aircraft 10 is about to, or has, takenoff but that the method will not be able to provide alerts that requirea comparison of responses received from an aircrafts avionics with theflight plan corresponding to that aircraft's current flight.

As shown in FIG. 6, the computerised method may additionally comprisereceiving 70, at the input module, aircraft location information, flightplan data and Controller-Pilot Data Link Communications messagescorresponding to the aircraft 10 and determining 72, based the aircraftlocation information and the flight plan data, if the aircraft hasdeviated vertically or laterally from the flight plan data by a givenamount. If the aircraft 10 is determined to have deviated by the givenamount, or more, then the method further comprises determining 74, atthe processor 22, if the deviation from the flight plan is authorised inthe content of the Controller-Pilot Data Link Communications messages.

If the aircraft 10 is determined to have deviated vertically orlaterally from the flight plan data by the given amount and thedeviation has not been authorised in the content of the Controller-PilotData Link Communications messages then the method comprises generating76, at an alerting module 30, an alert.

In further embodiments, the method may comprise generating an alert ifthe aircraft location information indicates than the aircraft 10 is in agiven region of airspace. Optionally, the alert may be generated if theflight plan data indicates that aircraft is estimated to intersect sucha given region of airspace in the future.

The method may further comprise processing received messages todetermine if a received data signal response is an emergency report or aconnection denial message. If a received data signal response isdetermined by the method to be an emergency report or a connectiondenial message then an alert is preferably generated, at the alertingmodule. This will highlight emergency situations or situations wherein aFANS connection has been unsuccessful respectively.

The method may further comprise storing received CPDLC messages and/orgenerated alerts in a data store 34, wherein the CPDLC messages and/orgenerated alerts may be retrieved from the data store and provided to auser upon a user request.

With reference to FIG. 7, a computerised method for monitoring thestatus of an aircraft may receive 78, at the input module, Air TrafficServices Facilities Notification (AFN) messages corresponding to theaircraft and receive 80, at the input module, aircraft locationinformation corresponding to the aircraft. The method of FIG. 7 furtherdetermines 82, at a processor, if one or more AFN message criteria havebeen satisfied by the received AFN messages and generates 84, at analerting module, an alert based on the determination and the aircraftlocation information.

For example, the AFN message criteria may specify that a given region orsector of airspace is monitored by a FANS enabled ANSP and thataccordingly it would be expected that a FANS enabled aircraft enteringthat sector of airspace would have an active FANS session with therelevant ANSP. If a FANS logon confirmation message (AFN Acknowledgementmessage) has not been received by the time the aircraft enters thesector, or after the aircraft has been in the sector for a given timeperiod, then the method may involve generating a corresponding alert.

Similarly, if a FANS enabled aircraft 10 has an active FANS session withthe ANSP of the sector that the aircraft is currently in and theaircraft is approaching another FANS enabled ANSP, it would be expectedthat the current ANSP 21 would send an AFN Contact Advisory message tothe aircraft 10 to identify the next ANSP 21 a and initiate theautomatic handover process.

If the method determines that an AFN Contact Advisory message has beensent to the aircraft 10, but that a FANS logon confirmation message hasnot been received within a given time period, then the AFN messagecriteria may specify that the method should proceed to generate anappropriate alert in order to highlight to the user that that automatichandover process between ANSPs does not appear to have been successful.

Each block in the flowcharts described above may represent a modulecomprising one or more executable computer instructions, or a portion ofan instruction, for implementing the logical function specified in theblock. The order of blocks in the diagram is only intended to beillustrative of an example. In alternative implementations, the logicalfunctions illustrated in particular blocks may occur out of the ordernoted in the figures. For example, the processed associated with twoblocks may be carried out simultaneously or, depending on thefunctionality, in the reverse order. Each block in the flowchart may beimplemented in software, hardware or a combination of software andhardware.

Front end software may be provided to facilitate the interface betweenthe user of the end user system 31 and the system 20. The front endsoftware may provide a geographical mapping display with monitoredaircraft depicted with corresponding icons. The front end software maydisplay visual and/or audible alerts to the end user when a generatedalert is output by the alerting module 30. Events such as AFN logons andconnection denial messages may be shown visually on the geographicalmapping display at the location that the event occurred.

A graphical user interface of the front end software may also allow theuser to review alert information, cancel, ignore or delete a given alertor retrieve CPDLC messages exchanged around the time of the alert or theevent that caused the alert to be generated. In one embodiment, thisinformation may be accessed by clicking on the icon corresponding to agiven aircraft 10. Furthermore, the front end software may comprise aconfigurable administration function that allows the end user to tunethe parameters of an ADS contract in order to adaptively balance thecost and performance impacts of the corresponding ACARS messaging for agiven flight or aircraft 10.

The graphical user interface may include an icon associated with amonitored aircraft 10 or the icon for the aircraft may be a specifiedcolour to indicate that the aircraft has an active FANS session.Furthermore, position reports for each aircraft 10 may be identified bytheir source, for example as ADS-C position information rather thanstandard ACARS position information or any other data source that isincluded, such as ADS-B or radar data. It will be appreciated thatdifferent colours may be used to identify the data source of a givenitem of position information for an aircraft 10.

1. A computerised method for monitoring the status of an aircraftcomprising: sending, from an output module of an aircraft monitoringsystem, a reporting contract request to the aircraft's avionics, thereporting contract request defining one or more report criteria uponwhich the aircraft's avionics are required to provide a data signalresponse to the aircraft monitoring system; monitoring, at an inputmodule of the aircraft monitoring system, for received data signalresponse sent from the aircraft's avionics to the aircraft monitoringsystem; determining, at a processor of the aircraft monitoring system,if one or more alert criteria have been satisfied by received datasignal responses; and generating, at an alerting module of the aircraftmonitoring system, an alert based on the determination.
 2. Acomputerised method according to claim 1, wherein the report criteriadefines a first interval at which the aircraft's avionics are requiredto provide a data signal response, the alert criteria defines a secondinterval and wherein an alert is generated, at the alerting module ofthe aircraft monitoring system, in the absence of a required data signalresponse being received during the first or second interval.
 3. Acomputerised method according to claim 1, wherein the report criteriaindicates an immediate data signal response is required from theaircraft's avionics and wherein an alert is generated, at the alertingmodule of the aircraft monitoring system, in the absence of a datasignal response being received within an interval defined by the alertcriteria.
 4. A computerised method according to claim 1, wherein the oneor more report criteria and the one or more alert criteria are receivedfrom a criteria database of the aircraft monitoring system and whereinthe data signal response comprises aircraft location informationcorresponding to the aircraft.
 5. A computerised method according toclaim 4, wherein the input module of the aircraft monitoring system isconfigured to receive flight plan data and ACARS data corresponding tothe aircraft and further wherein an alert is generated, at the alertingmodule of the aircraft monitoring system, if it is determined that aflight phase change message corresponding to the aircraft has beenreceived but flight plan data corresponding to the aircraft has not beenreceived.
 6. A computerised method according to claim 4, furthercomprising: receiving, at the input module of the aircraft monitoringsystem, flight plan data corresponding to the aircraft; wherein an alertis generated, at the alerting module of the aircraft monitoring system,if the aircraft location information is determined to deviate verticallyor laterally from the flight plan data by a given amount.
 7. Acomputerised method according to claim 6, further comprising: receiving,at the input module of the aircraft monitoring system, Controller-PilotData Link Communications (CPDLC) messages corresponding to the aircraft;wherein the alert is only generated by the alerting module of theaircraft monitoring system if the deviation from the flight plan data isdetermined, by the processor, not to be authorised in the content of theCPDLC messages.
 8. A computerised method according to claim 4, furthercomprising: receiving, at the input module of the aircraft monitoringsystem, flight plan data corresponding to the aircraft; wherein an alertis generated, at the alerting module of the aircraft monitoring system,if the aircraft is determined or estimated to intersect a given regionof airspace.
 9. A computerised method according to claim 8, wherein thegiven region of airspace is determined by a user selection or by aweather alert.
 10. A computerised method according to claim 8, whereinan alert is generated, at the alerting module of the aircraft monitoringsystem, if it is determined, at the processor, that a received datasignal response is an emergency report.
 11. A computerised methodaccording to claim 1, further comprising: receiving, at the input moduleof the aircraft monitoring system, Controller-Pilot Data LinkCommunications (CPDLC) messages corresponding to the aircraft; andstoring the CPDLC messages in a data store; wherein the CPDLC messagesare provided to a user upon a user request.
 12. A computerised methodaccording to claim 1, wherein an alert is generated if the data signalresponse to the reporting contract request is determined, at theprocessor of the aircraft monitoring system, to be a connection denialmessage.
 13. A computerised method according to claim 1, wherein thereporting contract request and the corresponding data signal responseconform to an Automatic Dependent Surveillance Contract.
 14. Acomputerised method according to claim 1, wherein the alerts that havebeen generated at the alerting module of the aircraft monitoring systemare stored in a data store and are provided to a user upon a userrequest.
 15. A computerised method according to claim 4, furthercomprising: receiving, at the input module, Air Traffic ServicesFacilities Notification (AFN) messages corresponding to the aircraft;determining, at the processor, if one or more AFN message criteria havebeen satisfied by the received AFN messages; and generating, at thealerting module, an alert based on the determination.
 16. A computerisedmethod for monitoring the status of an aircraft comprising: monitoring,at an input module of an aircraft monitoring system, for Air TrafficServices Facilities Notification (AFN) messages corresponding to theaircraft; receiving, at the input module, aircraft location informationcorresponding to the aircraft; determining, at a processor of theaircraft monitoring system, if one or more AFN message criteria havebeen satisfied by the received AFN messages; and generating, at analerting module of the aircraft monitoring system, an alert based on thedetermination and the aircraft location information.
 17. A computerisedmethod according to claim 16, wherein the AFN message criteria identifya time period for receiving a FANS logon confirmation message andwherein an alert is generated, at the alerting module, if the FANS logonconfirmation message has not been received after the aircraft has beenin FANS enabled airspace for the identified time period.
 18. Acomputerised method according to claim 16, wherein the AFN messagecriteria identify a time period for receiving a FANS logon confirmationmessage and wherein an alert is generated, at the alerting module, ifthe FANS logon confirmation message has not been received within theidentified time period from an AFN contact advisory message having beensent to the aircraft.
 19. A system for monitoring the status of anaircraft comprising: an output module configured to send a reportingcontract request to the aircraft's avionics, wherein the reportingcontract request defines one or more report criteria upon which theaircraft's avionics are required to provide a data signal response tothe system; an input module configured to receive data signal responsessent from the aircraft's avionics to the system; a processor configuredto determine if the one or more alert criteria have been satisfied byreceived data signal responses; and an alerting module configured togenerate an alert based on the determination.
 20. A system according toclaim 19, wherein the report criteria defines a first interval at whichthe aircraft's avionics are required to provide a data signal response,the alert criteria defines a second interval and wherein the alertingmodule is configured to generate an alert in the absence of a requireddata signal response being received during the first or second interval.21. A system according to claim 19, wherein the report criteriaindicates an immediate data signal response is required from theaircraft's avionics and wherein the alerting module is configured togenerate an alert in the absence of a data signal response beingreceived within an interval defined by the alert criteria.
 22. A systemaccording to claim 19, wherein the data signal responses that the inputmodule is configured to receive comprise aircraft location informationcorresponding to the aircraft.
 23. A system according to claim 22,wherein the input module is further configured to receive flight plandata and ACARS data corresponding to the aircraft and wherein thealerting module is configured to generate an alert if it is determinedthat a flight phase change message corresponding to the aircraft hasbeen received but flight plan data corresponding to the aircraft has notbeen received.
 24. A system according to claim 22, wherein the inputmodule is further configured to receive flight plan data correspondingto the aircraft; and wherein the alerting module is configured togenerate an alert if the processor determines that aircraft locationinformation has deviated vertically or laterally from the flight plandata by a given amount.
 25. A system according to claim 24, wherein theinput module is further configured to receive Controller-Pilot Data LinkCommunications (CPDLC) messages corresponding to the aircraft; andwherein the alerting module is further configured to generate the alertonly if the deviation from the flight plan data is determined by theprocessor not to be authorised in the content of the CPDLC messages. 26.A system according to claim 22, wherein the input module is furtherconfigured to receive flight plan data corresponding to the aircraft andwherein the alerting module is configured to generate an alert if theprocessor determines or estimates that the aircraft will intersect agiven region of airspace.
 27. A system according to claim 26, whereinthe definition of the given region of airspace is received from a userselection or a bad weather alert.
 28. A system according to claim 19,wherein the alerting module is configured to generate an alert if theprocessor determines that a received data signal response is anemergency report.
 29. A system according to claim 19, wherein the inputmodule is further configured to receive Controller-Pilot Data LinkCommunications (CPDLC) messages corresponding to the aircraft; andwherein the processor is further configured to store the CPDLC messagesin a data store and to provide the CPDLC messages to a user upon a userrequest.
 30. A system according to claim 19, wherein the alerting moduleis further configured to generate an alert if the data signal responseto the reporting contract request is determined by the processor to be aconnection denial message.
 31. A system according to claim 19, whereinthe reporting contract requests, that the output module is configured tosend, and the corresponding responses, that the input module isconfigured to receive, conform to an Automatic Dependent SurveillanceContract.
 32. A system according to claim 19, further comprising a datastore, wherein the processor is further configured to store the alertsthat have been generated in the data store and to provide the alertsthat have been generated to a user upon a user request.
 33. The systemof claim 22, wherein the input module is further configured to receiveAir Traffic Services Facilities Notification (AFN) messagescorresponding to the aircraft; and wherein the alerting module isconfigured to generate an alert if the processor determines that an AFNconfirmation message has not been received when expected based on theaircraft location information.
 34. A system for monitoring the status ofan aircraft comprising: an input module configured to receive AirTraffic Services Facilities Notification (AFN) messages corresponding tothe aircraft and aircraft location information corresponding to theaircraft; a processor configured to determine if one or more AFN messagecriteria have been satisfied by the received AFN messages; and analerting module configured to generate an alert based on thedetermination and the aircraft location information.
 35. A systemaccording to claim 34, wherein the AFN message criteria identify a timeperiod for receiving a FANS logon confirmation message and wherein thealerting module is configured to generate an alert if the FANS logonconfirmation message has not been received after the aircraft has beenin FANS enabled airspace for the identified time period.
 36. Acomputerised method according to claim 34, wherein the AFN messagecriteria identify a time period for receiving a FANS logon confirmationmessage and wherein the alerting module is configured to generate analert if the FANS logon confirmation message has not been receivedwithin the identified time period from an AFN contact advisory messagehaving been sent to the aircraft.